The present invention relates to an optical scanning apparatus which defectively scans a light beam, and an image forming apparatus mounted with the optical scanning apparatus.
As a color image forming apparatus such as a color laser printer, there is known a color image forming apparatus of a tandem type, which obtains a color image by writing, with a plurality of optical scanning devices by the use of laser beams, respectively, a plurality of pieces of information each having a color different from each other onto a plurality of photoreceptors, each of which is driven to be rotated, to form electrostatic latent images, developing the electrostatic latent images into developed images having different colors with a plurality of developing devices, respectively, and overlapping the developed images onto a transfer body to transfer the developed images thereon.
The optical scanning devices emit laser beams from semiconductor lasers which are controlled to be driven in accordance with retrieved image information signals of respective colors, respectively. Each of the laser beams is condensed, through optical components such as a polygon mirror, a lens and so forth, onto a surface of the corresponding photoreceptor which is uniformly charged, and scanned in a main-scanning direction. Thereby, the surfaces of the respective photoreceptors are written with image signals by scanning-beams, and hence, the electrostatic latent images are formed.
In an optical scanning apparatus used in the tandem color image forming apparatus, a color shift due to a registration position gap or to a misregistration in a sub-scanning direction is generated when timings of writing the electrostatic latent images, formed on the photoreceptors, are not accurately matched with each other for the respective colors.
In addition, the color shift is generated thus degrading the quality of an image when there is a difference in gradients, bending or the like of scanning lines of the laser beams in the plural optical scanning apparatuses provided corresponding to the respective colors.
Furthermore, the optical components are so disposed that the laser beams, going toward the corresponding photoreceptors, travel along pathways which are different from each other, respectively. Accordingly, there is a problem in that a scanning imaging lens, for example, is deformed by heat due to influences such as environmental temperature in which the color image forming apparatus is placed, rise in temperature within the apparatus, and so on, and thus, positions of the laser beams tend to vary easily. This phenomenon is notable when the lens is made of a resin.
In order to cope with such a shift in the scanning position, a correction is performed by detecting a detection pattern of the registration position gap, recorded on the transfer body, periodically, for example, at a time of start-up of the apparatus, between jobs and so on. However, there is also a problem in that the color shift gradually increases when the number of printed sheets for one job is large, since the scanning position further varies by generation of heat of a fixing device, a polygon motor and so forth, in accordance with a continuous printing operation.
To solve the above problems, various inventions have been proposed. For example, Japanese patent application publication No. 2003-337294 discloses an optical scanning apparatus and an image forming apparatus mounted with the optical scanning apparatus. The disclosed optical scanning apparatus includes a mechanism for adjusting scanning speed uniformly, and a mechanism for adjusting the inclination of scanning lines of laser beams. The scanning speed uniformity adjusting mechanism displaces reflecting mirrors, which guide laser beams corresponding to respective colors to photoreceptors, perpendicularly to a main-scanning corresponding direction and around an axis parallel with reflection surfaces of the mirrors, so as to adjust the scanning speed of the laser beams uniformly. The scanning line inclination adjusting mechanism displaces optical elements, which correct positions of the scanning lines of the laser beams in a sub-scanning corresponding direction, perpendicularly to the main-scanning corresponding direction and around an axis perpendicular to the sub-scanning corresponding direction. The disclosed optical scanning apparatus adjusts uniformity of the scanning speed with high precision, and is thus capable of obtaining an image that is excellent in absolute positional accuracy.
However, the invention disclosed in No. 2003-337294A detects positions in the sub-scanning direction of the laser beams by detecting toner marks on an intermediate transfer belt. Accordingly, there are problems in that not only can the detection not be performed constantly, including a case in which an image is being outputted, but also an accuracy of the detection, for example, line width varies, quality of an toner image tends to be varied due to humidity, etc., is low. Further problem is that a toner for the marks is consumed uneconomically for each detection.
Japanese patent application publication No. 2005-37575 also discloses an optical scanning apparatus and a color image forming apparatus. In the disclosed optical scanning apparatus, laser beam detectors for detecting positions in a sub-scanning direction of laser beams are arranged, and light-receiving elements which detect the laser beams are separated in a main-scanning direction. In addition, side edges of light-receiving faces, adjacent to each other, of the separated light-receiving elements are arranged at an angle in the sub-scanning direction. According to this invention, the light-receiving elements are disposed adjacently parallel to the main-scanning direction, so that there is an advantage in that accuracy in detection in the sub-scanning direction is unlikely to be influenced even when an amount of light of the laser beams is changed.
The invention disclosed in Japanese patent application publication No. 2005-37575 detects scanning lines of the laser beams to correct the positions in the sub-scanning direction of the scanning lines based on a result of the detection. However, since the laser beam detectors disclosed in No. 2005-37575A are not disposed at locations having a correlation with the laser beams which scan surfaces to be scanned, optimization of the positions at which the laser beam detectors are disposed has not been achieved. Thus, there is a possibility in No. 2005-37575A that the result of the detection of the positions in the sub-scanning direction of the laser beams detected by the laser beam detectors becomes different from the positions in the sub-scanning direction of the laser beams on the surfaces to be scanned, and thereby, the correction of the positions in the sub-scanning direction of the laser beams is erroneously performed. Therefore, it is likely that degradation of an image occurs, in contradiction to an improvement of quality of the image.